A very common need in electronics is converting AC powerline energy to DC power to supply an electronic circuit as a load. In addition, it is often necessary to regulate the DC power: that is, to maintain the load voltage approximately constant in spite of variations in the powerline voltage and the load current. Series regulators--in effect, controllable resistances in series with the load--have given place, in many applications, to switching regulators. In these regulators, the powerline voltage is rectified to DC, which is then switched into various inductors and capacitors at a frequency hundreds or thousands of times higher than the powerline frequency. These reactances alternately absorb powerline energy and deliver it to the load in a manner which is controlled to provide a constant load voltage.
A major reason for the popularity of switching regulators is that their power conversion efficiency can approach 100%, a figure unobtainable with series regulators.
Standard switching regulator topologies include the buck regulator, in which the output DC voltage is less than the input, and the boost regulator, whose output voltage is higher than the input. The single-ended primary inductance converter (SEPIC) is a more recent topology which uses two inductors and which has the considerable advantage of allowing the output voltage to be either higher or lower than the input.
A good discussion of the theory of operation of the SEPIC converter and its advantages and limitations is found in the paper "High power factor preregulator using the SEPIC converter", by Lloyd H. Dixon, Jr., which is included herein by reference. The paper is found in the Unitrode Power Supply Design Seminar Manual, copyright 1993 and published by Unitrode Integrated Circuits, Merrimack, N.H.
One problem which must be solved for most regulators is how to isolate the human operator (of the electronic circuit) from any possibility of direct contact with the AC powerline. A common solution is to use a transformer in the line-to-load path, either at the line frequency, or, preferably, at the higher switching frequency. But the transformer, while providing ohmic (DC) isolation, must also have a limited primary-secondary capacitance, so that line frequency leakage current which could flow through this capacitance will be restricted to a safe value.
As Dixon explained on page 6-11, there are technical reasons why it is difficult to include a transformer at the switching frequency in a SEPIC converter. He further added that there were no known solutions to the difficulties and that a solution, if found, would enhance the usefulness of the SEPIC topology.
In the present invention, powerline isolation in a SEPIC converter is achieved without using a transformer. Referring to FIG. 1, the converter is shown, for explanatory clarity, as comprising a primary circuit 20 coupled to a secondary circuit 30. The forward paths (22,32) of these circuits are connected with a capacitor Cc1, while their return paths (24,34) are connected with a low ohmic conductor, such as a wire. Clearly, the return path connection could subject one who touches the load 40 to a dangerous contact with the powerline supplying the source energy Vi and Ii. (There can also be a path of mutual inductance between L1 and L2, but this path would provide isolation at line frequencies.) In the invention, explained in more detail below, a second coupling capacitor Cc2 substitutes for the conductor in the return path (24,34), thus providing ohmic isolation for the output. If the series equivalent capacitance of Cc1 and Cc2 are chosen to equal the previous value of Cc1, there is no change in the high frequency operation of the circuit. Likewise, the parallel combination of the two coupling capacitors may be chosen to limit the leakage current to a safe value.